High transmittance, low emissivity coatings for substrates

ABSTRACT

The present invention provides a coating for a transparent substrate which exhibits a neutral color through a wide range of angles of incidence a light. The coating employs a base coat adjacent to the transparent substrate having a thickness of no more than about 275 Angstroms and may include two reflective metal layers having an intermediate layer of an anti-reflective metal oxide therebetween and an outer anti-reflective layer of metal oxide over the second reflective metal layer. If so desired, the coating of the invention may include an abrasive-resistant overcoat as its outer most layer. This overcoat is desirably formed of an abrasive-resistant metal oxide, such as zinc oxide, applied at a thickness which does not significantly affect the optical properties of said coated substrate.

FIELD OF THE INVENTION

The present invention generally relates to coatings for substrates andparticularly to substantially transparent coatings for transparentsubstrates such as glass.

BACKGROUND OF THE INVENTION

High visible transmittance, low emissivity coatings applied totransparent substrates such as glass are characterized by their abilityto transmit visible light while minimizing the transmittance of otherwavelengths of light, such as light in the infrared spectrum. Thischaracteristic is particularly useful for reducing radiative heattransfer without impairing visibility, such as in architectural glass orautomobile windows. For aesthetic reasons, in many such applications itis important to maintain reflection relatively consistent throughout thevisible spectrum so that the coating has a "neutral" color, i.e., isessentially colorless.

Generally speaking, such high transmittance, low emissivity coatingscomprise a "film stack" having at least one thin metallic film or layerwith high infrared reflectance and low transmissivity disposed betweenmetal oxide layers. The metallic layer may be virtually any reflectivemetal, such as silver, copper or gold, with silver being the mostfrequently-used metal for this application due to its relatively neutralcolor. Metal oxides useful in high transmittance, low emissivity filmsincluding oxides of titanium, hafnium, zirconium, niobium, zinc,bismuth, indium and tin. Prior art systems have also employed oxides ofmetal alloys, such as zinc stannate or oxides of indium/tin alloys.

The metal oxide layers of such coatings serve two important functions.First, they serve to reduce the visible reflection of the film stack,enhancing transmittance. The metal oxides used in these layers shouldhave a relatively high index of refraction, e.g., on the order of 2.0 ormore, in order to achieve this end. According to commonly acceptedprinciples in the art, the metal oxide layer between the transparent(e.g., glass) substrate and the first metallic layer must be at least300 Angstroms (A) in order to obtain a neutral, high transmittingcoating; such layers more commonly are between about 400 and 700 A inthickness.

Second, the metal oxide layers should serve to protect the reflectivemetal layer from the environment. Once such coated substrates areassembled and installed for use, the film stack is frequently isolatedfrom contact with the environment, as by disposing the film stackbetween two spaced panes of glass in a composite window structure.However, before these products are assembled, the film stack isfrequently subjected to relatively harsh conditions, such as byhandling, shipping or washing.

A variety of attempts have been made to enhance the ability of the metaloxide layers to protect the reflective metal layer in such film stacks.For instance, Gillery, et al. teach the use of titanium oxide as aprotective overcoat in U.S. Pat. No. 4,786,563, the teachings of whichare incorporated herein by reference. Although Gillery, et al. explainthat titanium oxides achieve the best results, they note that such anovercoat could be formed of a metal instead of a metal oxide; titaniumand alloys of iron or nickel are listed as prime candidates for such ametal layer. Gillery, et al. also teach that certain other oxides simplylack the requisite durability to be used as a protective overcoat. Zincoxide, bismuth oxide and tin oxide are all listed as having undesirableproperties, such as a lack of durability, which make them unsuitable fora protective overcoat.

However, it has been found that the use of a titanium oxide overcoatsuch as that taught by Gillery, et al. is particularly prone toscratching or abrasion during shipping and washing operations. Forinstance, when coated substrates are washed before being assembled intoa final product, the film stack comes into physical contact with awashing apparatus. It has been found that such a washing stage canphysically abrade an overcoat of titanium oxide or the like, noticeablydegrading the appearance of the finished article.

The particular composition of the layers of a film stack, as well astheir relative thicknesses, must be carefully chosen in order to achievethe desired properties in the coated substrate. As noted above, inaddition to maximizing visible transmittance while minimizingemissivity, in many instances it is desirable that a film stack have aneutral color. It has been possible to provide coatings for substrateswhich have an acceptable neutral color in transmission when viewed at anangle of incidence generally perpendicular to the plane of the film.However, as the angle of incidence is decreased, the film stack willtend to exhibit increasing color in transmission. It has been noted thatfilm stacks which are nearly completely neutral when viewed at aperpendicular angle will tend to exhibit a distinct, visible color whenviewed at a more acute angle. Windows of buildings or cars are viewedfrom various angles, of course, so variations in color of film stackscan frequently be detected in use.

It would be desirable to provide a film stack which serves as a highvisible transmittance, low emissivity coating for a substrate, yetremains substantially neutral at a wide range of angles of incidence.Additionally, it would be desirable to provide the film stack with aprotective coating which can withstand the rigors of normal handlingassociated with such substrates.

SUMMARY OF THE INVENTION

The present invention provides a high transmittance, low emissivity filmstack having a base coat formed of a metal oxide which is desirably nomore than about 275 Angstroms thick. This thickness is generallyconsidered in the prior art to be too thin to provide the necessaryantireflection between the substrate and the overlying reflective metallayer. We ave found, unexpectedly, that a metal oxide base coat formedat thickness of 275 A and below can adequately serve its antireflectivefunction. In addition to having obvious economic benefits due to thereduction in materials and energy necessary to apply the base coat,reducing the base coat in accordance with the present invention hassurprisingly been found to achieve significant improvements in the colorneutrality of the film stack. In particular, this relatively thin basecoat enables construction of a film stack that exhibits a neutral colorat a much wider range of angles of incidence than commonly associatedwith prior art film stacks.

In another embodiment of the present invention, a film stack including athin base coat of the invention also includes an outer layer of an oxideof a metal chosen from the group of zinc, tin, indium, or bismuth, or analloy including such a metal. This overcoat layer is applied at athickness which will not substantially affect the optical properties ofthe coated substrate if the overcoat is damaged or rubbed off. It hasbeen found that a film stack which includes an overcoat according tothis embodiment exhibits significantly enhanced physical durability ascompared to a film stack without such an overcoat.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a substrate coated with a film stack ofone embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A schematic representation of one preferred embodiment of a hightransmittance, low emissivity film of the invention is shown in FIG. 1.The film stack of the invention is applied over a substrate 10, whichmay be of any desired material. It is contemplated that a film stack ofthe invention, though, will be applied to a substantially transparentsubstrate, such as glass or a clear plastic material. The presentinvention has particular utility in coating glass panes for use aswindows in architectural or automobile related applications.

A base coat 20 of a metal oxide of the invention is applied over thesubstrate. It is contemplated that the metal oxide of this base coat 20be at least about 150 A thick, but should be no more than about 275 Athick. Alternatively, an optical thickness of this base coat iscontemplated as being between about 300 and about 550 Angstroms. Athickness of approximately 220 A is preferred for this layer.

Metal oxide layers analogous to base coat 20 of the present inventionemploy known metal oxides such as titanium oxide, hafnium oxide,zirconium oxide, zinc oxide, tin oxide, indium oxide, and bismuth oxide,as well as combinations of such oxides. These oxides are deposited bymagnetron sputtering. Magnetron sputtering techniques are well known inthe art and need not be discussed in detail here.

Although any one of the aforementioned metal oxides are useful for basecoat purposes, the thickness of the base coat may vary depending uponwhich of those metal oxides is chosen. If a metal oxide having arelatively low index of refraction (2.0 for zinc oxide) is used, a basecoat thickness of between about 150 Angstroms and about 275 Angstroms ispreferred. This yields a desired optical thickness (the product ofactual thickness and refractive index) of between about 300 Angstromsand about 550 Angstroms. If a material having a higher index ofrefraction is used, the actual thickness of the base coat 20 may bereduced while still maintaining the same desired optical thickness; iftitanium oxide, which may have an index of refraction of about 2.3-2.6,with about 2.4 being most common, is used, the preferred thickness ofthe base coat 20 is between about 125 and about 230 A, which yieldsabout the same optical thickness as that supplied by about 150-275 A ofzinc oxide.

As mentioned above, a film stack of the present invention also includesa reflective metal layer 30 applied over the base coat 20. Any suitablemetal layer which effectively reflects infrared radiation withoutsignificantly affecting visible transmittance is suitable for this metallayer. Although gold or copper could be used, as noted above, silver ispreferred because color neutrality of the film stack can be more readilyachieved. In the embodiment shown, in which two discrete reflectivemetal layers are employed as described below, a desirable inner (nearestthe substrate) reflective metal layer 30 has a thickness on the order ofabout 100 to about 150 A, with a thickness of approximately 125 A beingpreferred. However, a film stack of the invention may include only asingle reflective metal layer, omitting layers 40-50 of FIG. 1. In sucha single metal layer film stack, a layer of silver is preferably appliedat a thickness of about 100 to about 175 A, with a thickness ofapproximately 140 A being particularly preferred.

In the embodiment shown in FIG. 1, an outer reflective metal layer 50 isalso included. As is known in the art, it is desirable to include anintermediate layer 40 disposed between the inner and outer reflectivemetal layers (30, 50, respectively). The intermediate layer 40 may be ofany desirable composition; metal oxides which have been found to workwell for such an intermediate layer are substantially the same as thosewhich have proven effective for the inner metal oxide layer 20 set forthabove.

In the preferred embodiment shown in FIG. 1, though, two metal oxidelayers 42, 44 are used instead of a single intermediate layer 40. Inthis embodiment the silver layer is overcoated with a sacrificial layer42 of a metal such as titanium which is oxidized as the next oxidecoating in sequence is applied. The metal layer 42 thus serves as asacrificial layer to protect the underlying reflective layer 30 duringthe sputtering of the metal oxide layer 44. Applying the titanium metalat a thickness of approximately 20 A has been found to work well.

The metal oxide layer 44 may be formed of any suitable metal oxide, suchas an oxide of zinc, tin, indium or bismuth. According to one preferredembodiment of the present invention, the thickness of the intermediatemetal oxide layer 44 is at least about 3 times that of the base coat 20,i.e., the ratio of the thicknesses of the base coat 20 to theintermediate layer 40 is no more than about 0.33. For example, in a filmstack of the invention having a base coat 20 with a thickness ofapproximately 220 A, it is preferred that the intermediate layer 40 bebetween about 660 and about 900 A thick, with a range of about 700 toabout 750 A being preferred. This yields a ratio of the thickness of thebase coat 20 to that of the metal oxide layer 44 of between about 0.33and about 0.24, with a preferred range of about 0.31 (220 divided by700) to about 0.29 (220 divided by 750).

In the embodiment schematically depicted in FIG. 1, an outer reflectivemetal layer 50 is also employed. The metal of this reflective layer maybe the same as or different from the metal used in the inner reflectivemetal layer 30. The thickness of this second reflective metal layershould be selected such that the combined action of the inner and outerreflective metal layers achieves the desired visible transmittance andemissivity properties, as well as the color properties, for the filmstack. With an inner reflective layer 30 of approximately 125 A, is itpreferred that the outer reflective layer 50 be between about 125 andabout 175 A thick, with a thickness of approximately 150 A beingpreferred. Using such thicknesses for the inner and outer metal layers30, 50 has been found to achieve rather low emissivity while maintainingrelatively high visible transmittance and color neutrality. An outermetal oxide stack 60 is applied over the outer reflective metal layer50. It should be understood that this outer metal oxide stack 60 isemployed regardless of the number of reflective metal layers utilized inthe film stack. For example, if only a single reflective metal layer,such as layer 30 in FIG. 1, is employed, the outer oxide stack 60 may beapplied directly over that metal layer. Alternatively, if additionalreflective metal layers were to be employed, such as in a film stackwhich employs 3 or more reflective metal layers, the outer metal oxidestack 60 should be applied over the outermost of the reflective metallayers.

An outer metal oxide stack 60 according to the present invention may beof any suitable construction. In a preferred embodiment, the outer metaloxide stack 60 includes a barrier layer 62 applied directly to the metallayer 50, with one or more additional oxide layers being carried abovethe barrier layer. This barrier layer 62 may be substantially similar tothe barrier layer 42 described above in connection with the intermediatemetal oxide layer 40.

The outer metal oxide stack 60 desirably includes at least one metaloxide layer carried over the barrier layer 62 in order to minimize thevisible reflectance of the film stack and protect the integrity of theunderlying metal layer or layers. This metal oxide carried over thebarrier layer may be varied in composition and thickness to meet thespecific performance parameters desired for the film stack. Forinstance, a single layer of an oxide of titanium may be employed.However, in the preferred embodiment shown in FIG. 1, at least two metaloxide layers 64, 66 are disposed above the barrier layer 62.

It is desirable to form these two layers 64, 66 of different metaloxides rather than forming both layers of the same composition. One ofthe two layers 64, 66 preferably comprises an oxide of titanium,hafnium, zirconium or niobium or an alloy of such a metal, with titaniumoxide being particularly preferred. Such a layer provides additionalchemical resistance to the film stack, protecting the underlying metallayers from the attack of chemical agents. The thickness of this layercan be varied within a relatively broad range. It is preferred that thislayer be at least about 5-10 A thick in order to achieve an effectivechemical barrier, but this layer can be increased significantly beyondthat minimum thickness. Titanium, hafnium, zirconium and niobium sputterrelative slowly and oxide layers of such metals therefore tend to berelatively expensive to apply. It is therefore economically desirable toapply such an oxide layer at a thickness of no more than about 100 A. Ina preferred embodiment, this layer is formed of titanium oxide at athickness of a between about 50 and 100 A, with about 55- 60 A beingparticularly preferred.

The other of the two metal oxide layers 64, 66 in the outer stack 60 maybe formed of any desired metal oxide. It has been found that zinc oxide,tin oxide, indium oxide and bismuth oxide work quite well in thisapplication. In one preferred embodiment, this layer is formed of zincoxide at a thickness of between about 200 and about 400 A, with athickness of about 300 A being preferred. Although the relative order ofthe titanium oxide and zinc oxide layers may be varied, in thisembodiment zinc oxide is applied over a titanium oxide barrier layer 62as oxide layer 64 and titanium oxide is applied over the zinc oxidelayer as oxide layer 66.

It is also desirable to minimize the total thickness of this film stack.It is believed that minimizing the total thickness of this film stack,in combination with the use of a relatively thin base coat 20 asdescribed above, helps achieve a wider range of angles of incidencedisplaying a neutral color, as described more fully below. It iscontemplated that the total thickness of this film stack be no more thanapproximately 1700 A, with a total thickness of about 1600 A beingpreferred and a thickness of between about 1500-1600 A beingparticularly preferred.

Although the use of an inner metal oxide layer is well known, it isgenerally accepted that effective high visible transmittance, lowemissivity films having a neutral color require an inner metal oxidelayer at least 300 A thick. As noted above, the inner metal oxide layeris disposed between the substrate and the overlying reflective metallayer 30. A thickness of at least about 300 A has been thought necessaryto reduce reflection as light passes from the substrate to thereflective metal layer or vice-versa.

We have discovered that adequate antireflection can be obtained using asignificantly thinner base coat 20. Clearly, using less of the metaloxide material in forming this base coat will reduce the costsassociated with producing the coated substrate--not only are the costsof the raw materials reduced, but the power requirements andmanufacturing time necessary in applying such films are alsocorrespondingly reduced. Quite surprisingly, though, using less metaloxide material in the base coat does not significantly reduce theoptical properties of the coated substrate as commonly acceptedprinciples in the art would suggest. To the contrary, the use of a basecoat according to the present invention has been found to actuallyimprove the optical properties, particularly the color neutrality, ofthe resulting film stack.

As noted briefly above, it is important that coated substrates used incertain applications exhibit a neutral color. Much effort has beendevoted to selecting appropriate metal oxide and metal layers to achievea film stack having a neutral color. Colors of coated substrates areroutinely measured by using light which strikes the coated substrate atan angle substantially perpendicular to the glass pane. In performingsuch a test, a light source having a known wavelength distribution overthe visible spectrum is positioned on one side of the coated substrateand a detector is positioned on the other side of the substrate along aline which is substantially perpendicular to the surface of thesubstrate. The detector then measures the light intensity at variouswavelengths across the visible spectrum and compares these measurementsto the known wavelength distribution of the source to determine theoverall "neutrality" of the coated substrate. Such color tests are wellknown in the art and need not be explained in detail here.

It is important to note, though, that such tests are designed to measurethe color of light transmitted through the substrate at an anglesubstantially perpendicular to its surface. As the angle of incidence atwhich light strikes the film (alpha in FIG. 1) is reduced from 90°, thecolor of the coated substrate can change. Although this change can bequite difficult to quantify with current testing techniques, it has beennoted by those experienced in this field that this change can be quitesignificant. In particular, some coatings which have exhibited superiorneutrality when measured with light at a normal angle of incidence haveshown distinct, visible color, e.g., a yellowish tint, when viewed at amore acute angle of incidence.

However, in a coated substrate of the present invention the film stackssubstantially retain their neutrality over a wide range of angles ofincidence, e.g., at angles up to about 45°.

In an additional embodiment of the present invention, the base coat 20,reflective metal layers 30, 50, and the intermediate metal oxide layer40 (or layers 42 and 44) are substantially as described above. However,according to this embodiment the outer metal oxide stack 60 of theinvention includes an outer overcoat 70 applied as the outermost layerof the film stack, i.e., at the location disposed farthest away from thebase coat 20. The overcoat 70 is adapted to protect the underlyinglayers of the film stack from abrasions. In many instances, a substrateis coated at one location and must then be shipped to another locationfor assembly into a final product. For instance, glass panes may beprovided with a high transmittance, low emissivity coating at a coatingfacility and then be shipped to another remote facility for assembly toinsulative glass window units. Although it is possible to ship coatedsubstrates relatively short distances without unduly damaging coatings,when the substrates are shipped over long distances the product lossesdue to abrasion can be quite high.

For example, a glass product provided with a coating substantially asoutlined above and including titanium oxide as an outermost layer wastransported from a coating facility to an assembly plant less than 1,000miles away; nearly 80% of the shipment was considered to be unacceptablefor use in insulative glass units because of scratching and abrading ofthe coatings. By applying an overcoat of the invention to a secondshipment of glass provided with substantially the same film stack,though, the number of unacceptably coated panes was comparable to thatassociated with very short-range shipping and was within acceptable,expected ranges of manufacturing losses. This second shipment of glassincluding an overcoat showed very few scratches or other abrasions andwere visibly better than the prior shipment which omitted the overcoat.

The instant overcoat 70 desirably is formed of a material with a greatermechanical durability, i.e., a material which better resists abrasion orscratching, than the material immediately underlying this overcoat.Materials which exhibit relatively high mechanical durability whensputtered on as an overcoat of the invention include oxides of zinc,tin, indium, bismuth, or oxides of alloys including such metals, such aszinc stannate. Due to its relatively low cost and high sputtering rate,zinc oxide applied by magnetron sputtering of a zinc target in areactive oxygen atmosphere is particularly preferred.

Unfortunately, though, it has been found that such mechanically durablemetal oxide layers tend to be more susceptible to chemical attack thanother more conventional outer layers such as titanium oxide. When coatedglass panes are handled by workers assembling final glass products, theworkers often touch the film stack. Sweat on the workers' hands orchemicals used in the assembly process may contact the film stack andcan visibly mottle or otherwise degrade the film. Additionally, suchpanes are frequently washed before assembly; the chemicals in thewashing solutions can also adversely affect the film.

Therefore, it is desirable to apply an overcoat 70 of the invention at athickness which does not substantially affect the optical properties,and particularly the color, of the coated substrate. If a film stack ofthe invention is subjected to a harsh chemical environment whichdegrades the overcoat, the effects on the overcoat will notsignificantly affect the optical properties of the film or the coatedsubstrate. Even if the washing process were so harsh as to wash away theentire overcoat from the film stack, the optical properties of the filmwould not be substantially changed, but the overcoat would nonethelessserve to protect the underlying film from abrasion during handling,shipping or the like prior to washing.

The thickness at which an overcoat of the invention is applied will varyaccording to the metal oxide used; the maximum thickness of the overcoatwill be less for materials with a relatively high index of refractionthan for metals with a relatively low index of refraction. It has beenfound that an overcoat having an optical thickness of between about 10 Aand about 40 A can provide suitable abrasion resistance withoutsignificantly affecting the optical properties. Thus, an overcoat ofzinc oxide, which has an index of refraction of very near 2.0, isadvantageously applied at about 5 A to about 20 A thick.

While a preferred embodiment of the present invention has beendescribed, it should be understood that various changes, adaptations andmodifications may be made therein without departing from the spirit ofthe invention and the scope of the appended claims.

What is claimed is:
 1. A high transmittance, low emissivity coatedsubstrate comprising a substrate having a coating on a surface providingthe coated substrate with a substantially neutral color over a widerange of angles of incidence, the coating comprising:(a) A base coat ofan anti reflective metal oxide applied to a surface of said substrate ata thickness of no more than about 275 Angstroms; (b) A first reflectivemetal layer applied over said base coat; (c) An intermediate layer of ananti reflective metal oxide applied over said first reflective metallayer; (d) A second reflective metal layer applied over saidintermediate anti-reflective layer; and (e) An outer anti-reflectivelayer of a metal oxide carried over said second reflective metal layer.2. The coated substrate of claim 1 wherein the ratio of the thickness ofthe base coat to the thickness of the intermediate anti-reflective layeris no more than about 0.33.
 3. The coated substrate of claim 1 furthercomprising an abrasive-resistant overcoat as an outermost layer of thecoating, the overcoat being formed of a metal oxide having greatermechanical durability than the immediately underlying layer.
 4. A hightransmittance, low emissivity coated substrate comprising a substratehaving a coating on a surface providing the coated substrate with asubstantially neutral color, the coating comprising:(a) a base coat ofan anti-reflective metal oxide applied to a surface of said substrate ata thickness of no more than about 275 Å; (b) a reflective metal layerapplied over said base coat; (c) an antireflective layer of a metaloxide carried over said reflective metal layer; and (d) anabrasion-resistant overcoat as the outermost layer of the coating, theovercoat being formed of an oxide of a metal selected from the groupconsisting of zinc, tin, indium, bismuth, and alloys thereof applied ata thickness which does not substantially affect the optical propertiesof said coating.
 5. The coated substrate of claim 4 wherein the overcoatis an oxide of a metal selected from the group consisting of zinc, tin,indium and bismuth.
 6. The coated substrate of claim 4 wherein theovercoat is formed of zinc oxide.
 7. The coated substrate of claim 6wherein the zinc oxide is applied at a thickness of between about 5 Åand about 20 Å.
 8. The coated substrate of claim 4 wherein the metaloxide of the overcoat is applied at an optical thickness of betweenabout 10 Å and about 40 Å.
 9. A high transmittance, low emissivitycoated substrate comprising a substrate having a coating on a surfaceproviding the coated substrate with a substantially neutral color over awide range of angles of incidence, the coating comprising:(a) A basecoated of an anti-reflective metal oxide applied to a surface of saidsubstrate at a thickness of no more than about 275 Angstroms; (b) Afirst reflective metal layer applied over said base coat; (c) Anintermediate layer of an anti-reflective metal oxide applied over saidfirst reflective metal layer; (d) A second reflective metal layerapplied over said intermediate anti-reflective layer; and (e) An outeranti-reflective layer of a metal oxide carried over said secondreflective metal layer,the total thickness of the coating being no morethan about 1600 Angstroms.
 10. The coated substrate of claim 9 whereinthe total thickness is between about 1500 Angstroms and 1600 Angstroms.